The present invention relates to a cryogenic refrigerator.
In a superconducting device, it is necessary to cool a superconducting magnet section to a very low temperature. For this purpose, the superconducting magnet section is disposed, for example, in an enclosed vessel containing liquid helium, and cooled by the liquid helium within the liquid helium vessel to a temperature of approximately 4.2.degree. K. Further the liquid helium vessel is surrounded by an adiabatic casing, and the interior of the casing is maintained in a vacuum, so as to prevent heat of the atmosphere outside the adiabatic casing from transferring to the inside of the liquid helium vessel.
However, heat from the ambient atmosphere may transfer in the form of heat conduction via piping or in the form of heat radiation to the inside of the liquid helium vessel through the adiabatic casing. Such heat radiation may cause corresponding vaporization of liquid helium.
In order to cope with this problem, a cryogenic refrigerator is used to collect helium gas and reconvert it into liquid helium.
In such a cryogenic refrigerator, identical refrigerating circuits, each comprising an adsorbent, a counterflow heat Exchanger and a Joule-Thomson expansion valve, are serially connected together in a plurality of stages so as to create a cryogenic condition.
In each refrigerating circuit, the adsorbent is cooled and heated alternately. The adsorbent is heated to thereby release a coolant therefrom. Then, the coolant is sent to the Joule Thomson expansion valve to be expanded, and the Joule-Thomson effect provided at this time is utilized to lower the temperature of the coolant. Coolant whose temperature has been lowered in a preliminary stage is utilized to cool the adsorbent of the subsequent stage. A cryogenic condition is created in this way.
However, a conventional cryogenic refrigerator comprises a plurality of stages of identical refrigerating circuits, in all of which the adsorbent is used in a similar manner. As a result, the entire apparatus is inevitably large. This problem will be described in detail.
When activated carbon for example is used as the adsorbent the amount of adsorption increases as the temperature lowers, and this increase causes a corresponding increase in cooling ability. Conversely, the amount of adsorption decreases and, accordingly, cooling ability decreases, as the temperature rises.
Therefore, among the plurality of stages of refrigerating circuits, the quantity of the adsorbent used is to be smaller toward the final stage and larger toward the first stage. As a result, the increase in size and weight is particularly evident in those refrigerating circuits nearer the first stage. This makes it totally impossible to install the cryogenic refrigerator in a vehicle.
When the coolant is to be adsorbed by the adsorbent, the adsorbent is cooled by utilizing coolant resulting from the preliminary stage. On the other hand, when the coolant is to be released from the adsorbent, the adsorbent is heated by using a suitable heating fluid. For this purpose, it is necessary to intermittently supply the preliminary-stage coolant and the heating fluid to the adsorbent. Thus, a thermal switch, including an electromagnetic valve or the like, has to be disposed in the flow passage for the coolant and the heating fluid.
In order that the thermal switch be electromagnetically controlled at a high level of precision in a cryogenic environment, the solenoid section of the electromagnetic valve, etc. must be maintained at normal temperature. As a result, the solenoid section at normal temperature and the actuator section at a very low temperature are inevitably separated from each other. Disadvantageously, controllability deteriorates, and the electromagnetic valve becomes large in size. In addition, heat of the ambient atmosphere may transfer through the joint between the solenoid section at normal temperature and the actuator section at a very low temperature, thereby lowering operational efficiency.